1. Dirk Audring DE Academic Village Barcelona 12-15 May 2003 1 Supplying micro-grids with stationary fuel cell systems Prof. Dr.-Ing. G. Balzer, Dipl.-Ing. D. Audring Darmstadt University of Technology Department of Electrical Power Systems

2. Dirk Audring DE Academic Village Barcelona 12-15 May 2003 2 Structure Examined micro-grid Short-circuit performance Disconnection from power system Conclusions

3. Dirk Audring DE Academic Village Barcelona 12-15 May 2003 3 Examined micro-grid Micro-grid is supplied by one fuel cell Fuel cell sets voltage and frequency of micro-grid Induction motor and two loads are connected to micro grid Network connection is simulated for back-up power supply using a disconnecting switch

4. Dirk Audring DE Academic Village Barcelona 12-15 May 2003 4 Short-circuit performance Fuel cell acts as a voltage source with limited current output This leads to low short- circuit current at micro- grid Selectivity with over- current protection is difficult to achieve

5. Dirk Audring DE Academic Village Barcelona 12-15 May 2003 5 Short-circuit performance Short-circuit currents are limited by inverter Transformation ratio and vector group of inverter- transformer influences short-circuits at micro-grid

6. Dirk Audring DE Academic Village Barcelona 12-15 May 2003 6 Short-circuit performance

7. Dirk Audring DE Academic Village Barcelona 12-15 May 2003 7 Disconnection from power system Short-circuit at power system leads to voltage drop at connected micro-grid Induction motor slows down Induction motor must be re- accelerated after disconnection from network Power quality is poor during disconnection and not acceptable for PC etc.

8. Dirk Audring DE Academic Village Barcelona 12-15 May 2003 8 Conclusions Main characteristic of micro-grid supplied by fuel cell is low short-circuit current Selectivity with over-current protection is difficult to achieve Transformation ratio and vector group of inverter-transformer is to consider for calculation of short-circuit currents at micro-grid Disconnection from power system due to short-circuit has to carried out fast to reduce voltage disturbances at micro-grid